1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for reducing effects of 1) structural resonances in the control of mechanical structures, such as gimbaled turrets and 2) noise harmonics of time varying and/or uncertain frequency, such as the spin frequency noise in rate gyroscopes subject to power fluctuations.
2. Description of Related Art
In the control of mechanical structures, such as gimbaled turrets, the frequency of structural resonances must be known so they can be cancelled with notch filters in the controller to keep the feedback control loop stable. The present invention adaptively computes the frequency of the resonance in realtime during normal closed-loop operation of the controller to improve the accuracy of the frequency estimate and adapt to changes with time in the resonance frequency. With a rough initial estimate, the Realtime Adaptive Notch Compensator (RANC) of the invention solves for a more accurate estimate of the frequency and compensates the notch filter to stabilize the control loop thus minimizing oscillations due to the structural resonance.
Structural resonances are problematic in that they are very narrow in bandwidth, they vary from system to system even with identical structure design, and, in a given system, may also vary with gimbal position, temperature, and vehicle high-g maneuvers.
The conventional approach is to measure and characterize each system to determine the frequency at which structural resonances occur. Constant, dedicated notches are then placed at these frequencies to attenuate the resonance amplitude. These notches must be narrow in width; otherwise they will significantly degrade the controller's performance, in particular, the phase and gain stability margins. The emphasis on narrow notches increases the accuracy with which the structural frequency must be known, thus, making the measurement process lengthy and expensive.
The frequency measurement process can be extended to generate look-up tables of frequency versus gimbal position, temperature, vehicle acceleration, etc. These tables can then be implemented in an open-loop fashion in the controller to set the notch frequency depending on gimbal position, temperature, vehicle acceleration, etc. The open-loop nature, however, provides no feedback indication that the frequency was adjusted properly to minimize oscillations.
Another approach is to minimize the interaction of structural resonances with the controller by designing stiffer structures such that resonances occur at higher frequencies where they cannot be excited by the controller. The disadvantage is larger, heavier structures or structures built with expensive materials. Alternately, the controller bandwidth is kept low (at the expense of performance) such that the controller does not excite structural resonances.
Notch filters are also used to filter out noise harmonics in sensors such as spin frequency noise in rate gyroscopes. The RANC of the invention is also applicable in such application to compensate for variations in spin frequency due, for example, to temperature changes or power fluctuations.
The prior art approach is exemplified schematically in FIG. 1, wherein an input signal 10 to be measured is corrupted by sensor 12 with harmonic noise of constant frequency ωn, which is processed by notch filter 14 at fixed frequency ωn, resulting in filtered output 16 and notch output error 18. The notch output error 18 is calculated via summing node 19 as the difference between the true input signal 10 and the filtered output 16.